Electrical sound recording



Patented Dec. 5, 1939 UNITED STATES PATENT OFFICE 8 Claims.

In the electrical recording of sounds, in particular with the presentlmownelectro-mechanical sound record-cutting devices, when a record ismade on a disc or on a film, linear and nonlinear distortions areobserved which are due either to purely electrical factors, or to-mechanical factors.

For the former, record-cutting devices have been produced having anapparently constant impedance over the scale of recordable frequencies,but at the price of an extremely accurate manufacture. However,variations of temperature, of dampness, the ageing of certain partscause the electrical characteristics of the recordcutter to vary intime, by causing the impedance constant to vary.

For distortions due to factors of a mechanical nature, in spite of thegreatest care in manufacture, it has not yet been possible to produceperfectly linear record-cutting devices, since not only do all theseapparatus show distortions due to mechanical defects, but furthermore inall these aparatus of the same kind, resonance effects are observedwhich are due to the masses in vibration and particularly suchresonances vary with the nature of the support on which the recording iseifected, Thus, it is mechanically impossible to construct a recorderwithout a mechanical defect, without resonance, without variation withtime and which is linear for any support on which it records.

The object of the present invention is not only to eliminate all thesedrawbacks, but also to enable linear recording to be effected on anysupport.

For this purpose, all the mechanical or electrical effects which arecapable of causing distortion are corrected or compensated.

The invention is based on the principle that, in any recording, it isnot the characteristics of the current which is supplied to therecording member such as a cutter, which matter, but those of the recordsuch as the groove out.

My invention will be better understood from the following descriptiontaken inconjunction with the accompanying drawing, and its scope will bepointed out in the appended claims.

In said drawing:

Fig. l is a diagram of an example of an electrical sound recordingcutter.

Fig. 2 is a diagram of a modified cutter.

Fig. 3 is a diagram explaining the correction of the distortions.

Fig. 4 is a diagram of a cutter recording system.

Fig. 5 illustrates the application of the invention to photo-electricsound recording.

In Fig. 1, l is the permanent magnet, and 2 and '3 the pole shoes; P isthe armature carrying the recording needle and Z the coil receiving the5 modulated current, So far everything is pro-- vided as in the mostclassical cutter. But the armature P has an extension PX placed in acoil N which is in its turn placed in the magnetic field produced by thepole shoes 4 and 5. Any 10 vibration of the armature P, with itsextension PX produces a current in the coil N, exactly as 0 occurs in areproducing pick-up. At the ends of the coil N, there will therefore bea current which is proportional to the movements of the armature. 15 P.If no distortion is introduced when recording, the current sent into thecoil Z has exactly the same aspect as that issuing from the coil N.

In any case, the output current from the coil N has no connection withthe output current of the amplifier, but corresponds exactly to theaspect of the recorded groove. If, instead of a perfect sinusoid, suchas that sent into the coil Z, two truncated sine waves are recorded forany reason, the output from the coil N is not a sine wave, but twotrimcated sine waves,'since' the output current from the coil N reflectsthe aspect of the groove traced by the recording needle P.

The case of a recording with linear distortion will now be considered.In Fig. 4, a, transformer l, 2 Supplies an amplifier 3. It is assumedthat the power input to the amplifier 3 is absolutely constant and has afrequency which varies progressively over the scale to be recorded whichis shown by the power curve A (Fig. 3). At the 35 output end of theamplifier 3 there is therefore a response similar to that of the curve Aand this current is applied to the recording member 4 which it actuates.The two ends of secondary 2 of the input transformer I are alsoconnected to the input of the amplifier 5 and transmit thereto a currentwhich is equal to the input current of the amplifier 3. As the twoamplifiers 3 and 5 have the same characteristics, there is 45 at theoutput 6 a current which is absolutely equal to the output current ofthe amplifier 3, since said two amplifiers which have the same inputsource, operate in parallel. In other words, the response curve of thetwo amplifiers is the same (curve A of Fig. 3).

It is now assumed that there is a linear distortion in the recording.There is therefore an amplitude distortion in the recording member,either owing to an electrical phenomenon (resotortion introduced by therecording. For a con,-

, stantinput power, a similar curve to the curveA (Fig. 3) is thereforeobtained at the of the amplifier 5 Fi 4), and a curve to the curve B(Fig. 3) is found to have been recorded by the amplifier 3 (Fig. 4)

It is therefore obvious that the introduced by the recording is equal tothe difierence between the original curve A and the recording curve B.But the current induced in the coil N, as explained above, has the sameaspect as that shown by the curve B. Said current, which is calledauxiliary current, is then suitably amplified or reduced by I (Fig. 4)The output of I is connected to the of a transformer having twoprimaries 6 and 8.

The flux of said two primaries is elongated so as to induce in thesecondary a current which. is the result of the diiference of thecurrents which flow through the two primaries 6 and 8.

At the end of the secondary 9 there will therefore be a currentresulting from the diflerence between the curve A and the curve B. Saidcurrent corresponds to the amount of distortion introduced during therecording by the recormg member 4 and its electric Said current,

called pilot current, is conveyed to power am- In order to give a betterlmdg of this compensating current, reference will be had to the curvesof Figure 3. 7

When the current produced or by the recording member, after cation orreduction, is at the same level as the current of the amplifier 5,thecurve B has the same level as and is identical with the curve A, and asthe two fluxes of the two primaries are in opposition, the outputcurrent from thesecondary9 (Fig.3) lhitwhenflaecnrveB has a lower levelthan the curve A, there is in the secondary 9 a current in a which willbe called .positive, whereas when the curve B is at a higher level thanthe-curveAJhereis a current of opposite or negative sign in thes'econdary 9.

.'1'hus,assumingthatatthepointlofthecurvesBandA,thecurveBisatalevel-15db relatively to the curve A which isat 0 decibels, the current fi'omthesecondary .9 is amplified or reducedby the II and controls the amplification factor of the amplifier 3,increasing same by 15 db.

Owingto this fact, the currentattheoutmltof 'theamplifier3givesanincreaseofpowertothe recorder I, by'the amount 11 forthe wtrent produced in the coil N and amplified by I:

to be increased by 15 db., so that the cnrrmtflowingtliroughtheprimarydthesame I levelas that flowing through the. i.7

As soon as this level is reached, time n no longer any current in thesecondary S and, owing,

to this fact, the amplifier 3 again under.

normal conditions, since at this instant, the curve B coincides with thecurve awnemw any distortion will 75 Atthepoint2,ifowingtoanyeitectofmonaneeorforanyotherreasomtheamplifierl givesa-level which is higher than that of the original curve A, for'example,by 15 db, a cur-5 I rentisinducedinthecoilNwhichgivesintheprimarydacurrentoiwhichthestrengthis greater by 15 db. than that flowingthrough the primary 5.

. Owing to this fact, the current induced in 5 has a negative directionand reduces the ampliflcation factor of the amplifier 3 to the extentnecessary for the recording effected by 4 to give aninduced current inthe coil N of which the level is lower by 15 db. than that inducedtherein before correction, that is to say which brings backthecurveBtothesamelevel asthe curve A. And this result is obtainedwithout any inertia or dephasing, since the compensating action isinstantaneous and the correction is efl'ected as soon as the distortionarises.

The same system naturally corrects any nonlinear distortion,conditionally of course on dephasing being avoided between the inputcurrent of the secondary 2 and that of the secondary 9. But there is nodifliculty in obtaining this result, particularly if instead of usingtransformers'to place the current directly at the output of theamplifier in opposition to the current produced by the vibrations ofthe, armature carrying the recording needle, solely tubes are used. a

The detailed description is given solely by way of example, since it isobvious that the principle of the invention can be applied in variousmanner without exceeding the scope of the invention- Thus, instead of acoil such as N shownin Fig. 1, in order to avoid any direct induction,the extension of the armature may operate as the slider of a rheostat Raccording to the modification illustrated in Fig. 2. Or again, it may,compress a piezo-electric crystal so as to ob-- tain therefrom thenecessary voltages at the output; or again, this same armature may, withor of which the capacity varies proportionally to the position of thearmature. 'Ihus, the means for placing the current directly at theoutput of the amplifier in opposition with the current produced by thevibrations 6f the armature, may

be quite different from those illustrated as an example, withoutexceeding the scope of the invention. Thus this opposition of the twocurrents, instead of being eflfected by a transformer,

.maybeeflectedbyahibesystem,oragainby any other system notshown, withoutfor this reasondeparting from the principle 01' the invention.

Said invention, which 'is described in detail for a sound cuttingdevice, is consequently advanapplicable to any system for recording on adisc or on a flhn, either by cutting, or by optical means, since itsuilices forthe recording member to be in operation in order to detecttherein a form of distortion which is compensated by the systemillustrated. Thus,

films, illustrated in m. s, the light from the lamp A is condensed bythe optical system and'c impinges, in the shape of a beam of light, onthe recording galvanometer G. From the mirror of the latter a part ofsaid beam passes through the optical system SP and impinges on the filmF where it efiects the recording. Another part of the luminous beamwhich is reflected after modulation from the mirror of the galvanometerG, impinges on the prism L and is reflected on to the mirrors M and Nwhence it is reflected on to the photo-electric cell R. It is obviousthat the output current from the photo-electric cell R has absolutelythe same aspect as the sound recorded on the film F. The output currentfrom, the photo'- electric cell R, in this type of recording, therefore,replaces the current illustrated as the output current of the coil N ofFig. 1 or of the rheostat 2 of Fig. 2. Thus the analogy between the twosystems of recording is obvious, as'are likewise obvious the advantagesprocured by this novel system, for compensating any distortion,

in any recording 'system wherein the recording member is subjected toany movement.

The present invention therefore applies, not only to recording devicesoperating by cutting on discs or on films, but also to all recordingde-. vices, either electro-mechanical, or electro-optlcal wherein therecording member or element is subjected to an oscillatory or vibratorymovement.

I claim:

1. In the electrical recording of sounds, the 7 method which consists incausing the recording member itself to modulate an auxiliary electriccurrent which is proportional to the sound recorded with itsdistortions, placing said auxiliary current in opposition with a currentissuing directly from an amplifier connected in parallel with theamplifier piloting the recording mem-- ber, utilizing the result thusobtained from the two currents for piloting the amplification factor aof the amplifier jwhich actuates the recording member in order tocorrect all distortions introduced by the recording. member or by theelectricalcircuit of same. k

2. In the electrical recording of sounds, asystem comprising a source ofsupply, an input amplifieractuating a recording member, means forproducing an auxiliary ing said auxiliary current in opposition in areceiver with a current supplied direct by said source of supply throughthe intermediary of amplifying and transforming means, and means forconditioning the resulting output current that which'would havebeenproduced by the ,recording member proper.

4. A system according to claim 2, wherein the auxiliary current isproduced by an intermediate,

member giving a current which is proportional to that which would havebeen produced by the recording member proper.

tem comprising a source of light, conditioning current which is theimage of the recording effected, means for plac- 1 circuitof same; 5. Inthe electricalrecording of sormds, a sys-' means, a recording memberadapted to modulate the light, means whereby part of the modulated lightis caused to act on a film, and means causing another part of themodulated light to act on a photo-electric cell which produces-anauxiliary current which is the'ima'ge of the recordber, placing saidcurrent'in "opposition with a current obtained directly across theterminals {of the source actuating the recording member, utilizing thedifference thus obtained from the two said currents as a pilot currentfor piloting said auxiliary current and the current directly emanatingfrom the actuating source, andtherebycorrecting for'all the distortionsintroduced by the recording member or by the electrical circuit of same.

7. In the electrical recording of sounds, the m'ethod'which consists incausing the recording member itself to produce an auxiliary electriccurrent which is the exact image, of the sound the amplification factorof the amplifier. which actuates the recording member propor-' 'tionallyto the algebraical difierence between as recorded with all itsdistortions by said member, placing said current in. opposition with acurrent derived from the source before the amplifie utilizing thedifference thus obtained from the two said currents as a pilot currentfor piloting theamplification factorof the amplifier which 'actuates therecording member proportionally to the algebraical diflerence betweensaid auxiliary current and the-current directly emanating from theactuating source, and thereby correctingfor all the distortionsintroduced by the recording member or by the electrical circuit of same.

8. In the electrical-recording'of sounds, the method which consists incausing the recording member itself to produce an auxiliary electriccurrent whichis the exact image of the *sound as recorded with all itsdistortions by said memcurrent derived amplifier stage in which thecorrection is effected, utilizing the difference thus obtained from thetwo said currents as a pilot current for .piloting the amplificationfactor of the amplifier which actuates the recording memberpropertionally to the algebraical difference between by the recordingmember or rum nant-o nrccmsapr.

ber, placing said. current in opposition with a from the ,amplifierbefore the said auxiliary current and the current-- directly emanatingfrom the actuating source, and thereby correcting for all thedistortions introduced, by the electrical

